For generation of a reference frequency, a crystal oscillator is used in many cases. Although the crystal oscillator has a high Q factor and a high level of temperature stability of a resonance frequency, it is hard to integrate the crystal oscillator with a semiconductor integrated circuit. Meanwhile, a method for achieving high frequency accuracy comparable to that of the crystal oscillator without using the crystal oscillator is actively proposed.
For example, an oscillator using an electrostatic-type MEMS or a piezoelectric-type MEMS is proposed. Such the oscillator utilizes mechanical resonance or acoustic resonance, which is similar to the crystal oscillator in terms of operation principle. Moreover, such the oscillator is inherently more likely to achieve a high Q factor as compared with an LC resonator.
Even though integration of such a mechanical resonator or an acoustic resonator on a semiconductor substrate is possible in principle, it requires special material, structure, manufacturing process, implementation method and the like. Moreover, it is necessary to perform frequency tuning one by one due to variations in an oscillation frequency caused by manufacturing process variability. In this manner, there are many problems remained to be solved.
It is therefore desired to achieve a semiconductor device provided with an oscillator that is easy to be formed on a semiconductor substrate and has high frequency accuracy.